The owners of pre-loaded pneumatic air rifles now have to be licensed. A requirement to do so takes effect from 14 January.
Police say the firearms were increasingly the weapon of choice for criminals who previously did not have to undergo rigorous background checks needed for a licence.
Pneumatic air rifles were used to kill Sergeant Don Wilkinson in 2008 and Keith Kahi in July last year.
Inspector Joe Green says owners of the guns have had a grace period.
Firearms specialist Chris Ziesler of the Sports Industry Association says
the airguns were regarded as almost toys among the wider shooting community.
